This invention relates generally to the field of brake drums for motor vehicles, and specifically to the field of lightweight brake drums.
Brake shoe and brake drum-type brakes have been used on motor vehicles for many years. While many automobiles now use disc-type brakes, brake shoe and brake drum-type brakes are still used in many automobiles, and especially for braking the rear wheels in almost all heavy duty trucks.
The weight of a motor vehicle""s brake drums has become increasingly important to the vehicle manufacturer and to the vehicle operator. First of all, the weight of the vehicle""s brake drums affects the mileage efficiency of the vehicle. This factor is becoming increasingly important to the manufacturers of automobiles sold in the United States. The Federal Government is putting considerable pressure on automobile manufacturers to continuously increase the mileage efficiency of their automobile line. Automobile manufacturers are presently undertaking a frantic search to reduce the weight requirements on even the smallest automobile component.
The weight of brake drums is even more important to truck manufacturers. The weight of a truck""s brake drum not only affects the truck""s mileage efficiency but also directly affects the amount of cargo which can be transported by a truck. This stems from the fact that governmental regulations strictly limit the gross weight of all commercial vehicles. Thus, any savings in the weight of a commercial vehicle allows the owner of that vehicle to carry a like quantity of additional weight. In the highly competitive trucking industry, the total quantity of freight which can be transported per load is critical to profitability.
Conventional brake drums are manufactured from ductile iron, cast iron or steel. A typical large truck brake drum weighs about 120 pounds. Attempts have been made to reduce this weight by manufacturing the drums from lighter materials, such as aluminum and aluminum alloys. However, the use of lighter materials is restricted by strength requirements. For example, a typical truck brake drum must have an internal yield strength in excess of 40,000 psi. Brake drums constructed from aluminum and aluminum alloys alone do not have this high of an internal yield strength.
In an attempt to take advantage of lightweight materials while retaining adequate strength requirements, several attempts have been made to use brake drums made of a combination of lightweight and heavier materials. For example, in U.S. Pat. No. 1,989,211, a bimetallic brake drum is proposed which comprises a cast aluminum housing in combination with a steel internal liner. The resulting brake drum is lighter than conventional brake drums and has sufficient internal yield strength. However, such a drum is not fully satisfactory. For one thing, the steel liner must be relatively thick to provide for adequate wear life. For truck brake drums, the steel liner must be at least xe2x85x9c of an inch thick to obtain sufficient internal yield strength. This means the brake drum remains relatively heavy. For another thing, the internal liner has a strong tendency to slip within the outer housing. This requires that the liner/housing interface be provided with transverse ridges or spines to lock the liner within the housing. (See, for example, U.S. Pat. No. 1,989,211.) In practice, this generally means that the housing and liner must be cast together.
Accordingly, there is a need for a lightweight brake drum which is even lighter than the bimetallic lightweight brake drums of the prior art, a brake drum which does not require ridges or spines between the housing and the liner and a brake drum which does not require dissimilar brake drum components be cast in a single operation.
The present invention is directed to a brake drum which meets these needs. A brake drum according to the present invention includes a tubular inner member having an interior surface suitable for contacting a brake pad and an exterior surface, a length of wire snugly wrapped around a portion of the exterior surface, and at least one fastener for securing at least a portion of a wheel assembly to the brake drum. Preferably, the brake drum includes a tubular outer shell molded over and substantially covering the length of wire to protect the wire and provide additional support to the brake drum.
As described in detail below, the length of wire wrapped around the tubular inner member supports the inner member. Thus, the inner member and the outer shell of the brake drum can be made from similar, lightweight materials having lower internal yield strengths than the prior art steel brake drums. The term xe2x80x9cinternal yield strengthxe2x80x9d as used in this Application means the amount of internal pressure which the brake drum can withstand without failing.
Further, since the inner member and the outer shell can be made of similar materials with similar rates of thermal expansion, the outer shell can be molded over the wires, there is no need for ridges or spines between the inner member and the outer shell.
Preferably, multiple layers of the length of wire are wrapped around substantially the entire exterior surface to support the entire inner member. Preferably, the length of wire has a diameter of between about 0.1 inches and about 0.4 inches, has a tensile strength of at least 180,000 psi and is wrapped at a tension of at least about 25 foot-pounds to provide tight, consistent wrapping of the length of wire around the exterior surface and sufficient support of the inner member.
Since the length of wire supports the inner member and inhibits expansion of the inner member, the inner member and the outer shell can be made from lightweight materials having a density of less than about 0.15 pounds per cubic inch, such as aluminum and aluminum alloys. For example, the inner member can be made of an alloy which includes at least about seventy-five (75) volume percent aluminum and between about ten percent (10%) and about twenty-five percent (25%) abrasive material so that the brake pads can grip against the brake drum.
The invention is also a method for making a brake drum. The method includes manufacturing a tubular inner member and wrapping a length of wire tightly around an exterior surface of the tubular inner member. The method also can include molding an outer shell which substantially covers the length of wire around the exterior surface to provide additional support to the brake drum.
The present invention provides a strong, lightweight brake drum which can be manufactured relatively inexpensively since the inner member and the outer shell can be made from similar materials and there is no need for ridges and spines between the inner member since the outer shell can be molded over the wire.